OVERVIEW OF DIGESTIVE SYSTEM

OVERVIEW OF DIGESTIVE SYSTEMLecture 1Dr. Zahoor Ali Shaikh1DIGESTIVE SYSTEMThe primary function of the Digestive System is to transfer nutrients, water and electrolytes from the food into the body.Ingested food is source of energy from which cell generate ATP to carry out energy dependent activities e.g. active transport, contraction, secretion.2DIGESTIVE SYSTEMFood we take is digested or broken down into small molecules that can be absorbed from the GIT [Gastro Intestinal Tract] into circulatory system, for distribution to the cells.Normally 95% of ingested food is available for use by the body. Food is ingested digested absorbed distributed and used.3DIGESTIVE SYSTEMWe will have overview of Digestive System, we will discuss:Basic Digestive ProcessComponents of Digestive SystemRegulation of Digestive System 41. Basic Digestive Processes

Digestive System performs four basic processes:Motility SecretionDigestionAbsorption

We will discuss one by one.5Basic Digestive Processes

1. MotilityMotility means movements, muscle contraction that mix and move the food forward.Smooth muscle in GIT has - Tone [constant level of contraction] tone is important to maintain study pressure on the contents present in the GIT. - Action Potential which induces contraction

61. MotilityThere are two types of muscle contraction (i). Propulsive movements (ii). Mixing movements

(i). Propulsive movementThey propel or push or move forward the contents. Rate of propulsion varies in GIT.Why? Because it depends on the function of the region. E.g. movement of food is fast in esophagus, and slow in small intestine.71. Motility(ii). Mixing movementIt has two functions: - mixing the food with digestive juices to help in digestion of food - help in absorption of food due to exposing the contents of food to the absorbing surface of GIT.81. MotilityIn GIT, where smooth muscle is present, it is functioning by Involuntary mechanism. We have voluntary control in GIT only on chewing, swallowing, and defecation. Why? Because in these areas skeletal muscles are involved which are under voluntary control. 9Basic Digestive Processes2. SecretionBy exocrine glands (through ducts).Digestive secretion consist of water, electrolytes, enzymes, mucus, or bile salts.Secretion of digestive juices is active process (require energy). 10Basic Digestive Processes3. DigestionWe take three type of food: i). Carbohydrate ii). Proteins iii). FatsThese are large molecules, therefore, they are digested and then absorbed into the blood or lymph. Word Digestion means breakdown of complex food stuff into smaller, absorbable units by enzymes.11Basic Digestive Processes3. Digestioni). CARBOHYDRATESCarbohydrates are absorbed as monosaccharide e.g. glucose, fructose, and galactose.RememberCellulose [polysaccharide found in plants] can not be digested, therefore, works as bulk or ingestible fiber.

12Basic Digestive Processes3. Digestionii). PROTEINProtein are absorbed as - Amino acids - Small polypeptidesiii). FATFat are absorbed as - Monoglyceride [glycerol with one fatty acid] - Free fatty acid

13Basic Digestive Processes4. AbsorptionIn small intestine, digestion is completed and most absorption occurs.Water, vitamins and electrolytes are also absorbed.

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GIT ANATOMY16

GIT FUNCTIONS2. DIGESTIVE TRACT & ACCESSORY DIGESTIVE ORGANSDigestive TractIt is a tube about 4.5 meter [15 feet] in length.It includes mouth, pharynx [throat], esophagus, stomach, small intestine [duodenum, jejunum and ileum], large intestine [cecum, appendix, ascending, transverse, descending colon, rectum] and anus. NOTE they are continuous but there are regional modifications which allow them to specialize in digestive activity.17 2. DIGESTIVE TRACT & ACCESSORY DIGESTIVE ORGANS Digestive TractIt is continuous from mouth to the anus, the lumen of this tube is continuous with external environment.Contents in the lumen are technically outside the body.Only after the substance is absorbed from the lumen then it becomes the part of body. E.g. pH of stomach content is 2 due to presence of HCL, but body fluid pH compatible with life is 6.8 8.

18DIGESTIVE TRACT & ACCESSORY DIGESTIVE ORGANSACCESSORY DIGESTIVE ORGANS - Salivary glands - Pancreas [Exocrine] - Biliary system liver, gallbladderExocrine gland means organs lie outside the digestive tract and empty their secretion through ducts into the digestive tract lumen.

19DIGESTIVE TRACT WALLDigestive Tract Wall has four layers: 1. Mucosa 2. Sub-mucosa 3. Muscular layer 4. Serosa 2021

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DIGESTIVE TRACT WALL

1. MucosaIt lines the luminal surface.It is divided into three layers: - Mucus membrane - Lamina propria - Muscularis mucosaMucus membrane Inner epithelial layer, protective and specialized for absorption. It has exocrine gland cells for secretion of digestive juices. It has also endocrine gland cell for secretion of GIT hormones. 23DIGESTIVE TRACT WALL

1. Mucosa [cont]Lamina propria Middle layer of connective tissue. It has Gut associated lymphoid tissue [GALT] important in defense against intestinal bacteria. Muscularis mucosa Smooth muscle. Mucosal surface is highly folded which increases the surface area for absorption. 24DIGESTIVE TRACT WALL2. Sub-mucosa [under the mucosa]It is thick layer of connective tissue which provides distensibility and elasticity.It has large blood vessels and lymph.It has nerves known as SUB-MUCOSAL PLEXUS or MEISSNERS PLEXUS.

25DIGESTIVE TRACT WALL3. Muscular layerSmooth muscle coat of digestive tube which surrounds sub-mucosa.It has two layers: - Inner circular muscle layer - Outer longitudinal muscle layerContraction of inner circular layer decreases the diameter of lumen at the point of contraction.Contraction of outer longitudinal layer shortens the tube.26DIGESTIVE TRACT WALL3. Muscular layer [cont]Contraction of both layers [circular and longitudinal] causes propulsive and mixing movements. Another nerve network called Myentric plexus lies between the circular and longitudinal muscle layer.IMPORTANT Gut Local activity is regulated by sub-mucosal [meissners plexus] and Myentric plexus and also by hormones.

27DIGESTIVE TRACT WALL4. SerosaIt is outer connective tissue covering of digestive tract. It secretes watery, slippery fluid that lubricates and prevents friction between digestive organs and surrounding viscera.Serosa is continuous with the mesentery, which is attached to inner wall of abdominal cavity. 283. Regulation of Digestive System

Four factors regulate the digestive functions:1. Smooth muscle [autonomous] function2. Intrinsic nerve plexus [sub-mucosal and myentric]3. Extrinsic nerves 4. GIT hormones

293. Regulation of Digestive System

1. Smooth muscle [autonomous] functionSome smooth muscle cells are specialized and work as pace-maker cells called interstitial cells of CAJAL [like cardiac muscle cell].WHY? Because they have automatic changes in membrane potential. They are non-contractile cells. They lie between circular and longitudinal smooth muscle layers. 303. Regulation of Digestive System

1. Smooth muscle [autonomous] functionSelf-induced electrical activity in smooth muscle is slow wave potential or Basic Electrical Rhythm [BER]. Slow wave are below threshold level, if they reach threshold level, action potential will occur and lead to muscle contraction.313. Regulation of Digestive System

2. Intrinsic nerve plexus [sub-mucosal and myentric]They run throughout the GIT and have about 100 million neurons and do the self-regulation of intestine. They are called ENTERIC NERVOUS SYSTEM.ENTERIC NERVOUS SYSTEM it has sensory neurons which respond to local stimuli. It has motor neuron which supply smooth muscle, exocrine and endocrine cells, therefore, affect the motility, secretion, and GIT hormones.323. Regulation of Digestive System

3. Extrinsic nervesThese are ANS [Autonomic Nervous System].It originates outside the digestive tract and supplies the digestive organs.ANS sympathetic and para-sympathetic ANS affects motility, secretion by acting directly on smooth muscle or affecting the intrinsic plexus [sub-mucosal and myentric].Para-sympathetic nerve supply to GIT arise via VAGUS NERVE. It increases motility and secretion.333. Regulation of Digestive System

4. GIT hormonesIn Mucosa of some regions of Digestive tract are located endocrine gland cells.GIT hormones are carried by blood to other areas of GIT and cause excitation or inhibition of smooth muscle of GIT.

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RECEPTORS PRESENT IN GITThree types of sensory receptors are present which respond to local changes.(i). Chemoreceptors sensitive to chemical changes in lumen.(ii). Mechano receptors [pressure receptors] sensitive to stretch on the wall.(iii). Osmo receptors sensitive to Osmolarity of luminal contents.36RECEPTORS PRESENT IN GITStimulation of these receptors causes - Neural reflexes or Secretion of hormones which effect motility, secretion of digestive juices.In GIT, two type of reflexes occur: 1. Short reflexes local reflex in wall of digestive tract 2. Long reflexes reflex between CNS and Digestive system37Thank you38